The present invention relates to a variable-frequency magnetic coupling oscillator suitable for controlling an output voltage and current of a converter used in a battery charger for electric vehicles as well as of other general-use converters.
For charging secondary cells used as a power supply in an electric vehicle, a non-contact type battery charger has been proposed which makes use of an electromagnetic induction by a high-frequency transformer. The core of a battery charger is a so-called one converter in which a PFC (power factor correction) converter and a DC/DC converter are formed integrally. For controlling a plurality of switching elements, used in the converter, to turn on and off, it has been necessary to control the frequency of the high-frequency transformer for control of the battery charger output voltage since the output voltage varies greatly due to load current when the frequency is constant.
For this reason, the circuit controlling the plurality of switching elements incorporated in the one-converter circuit should be provided with a function of variable-frequency control. A flip-flop is driven with an output pulse of a V/F converter for variable-frequency control of the plurality of switching elements in the battery charger converter, as shown in FIG. 3.
As seen in FIG. 3, two switching elements 103 and 104 are connected in parallel across a primary coil 108 of a pulse transformer 107, and two secondary coils 109 and 110 of different polarities are provided at the secondary side of the pulse transformer 107. When driven, a V/F converter 101 delivers an output pulse to a flip-flop 102. The flip-flop 102 will deliver a set output signal and a reset output signal to the switching elements 103 and 104. These switching elements 103 and 104 are driven with these output signals from the flip-flop 102 to turn on and off alternately. Signals to turn on and off, alternately, the plurality of switching elements incorporated in the one-converter circuit of a battery charger are delivered from secondary coils 109 and 110 of the pulse transformer 107. However, the method for variable-frequency control using the above-mentioned V/F converter and flip-flop is disadvantageous in the complicated circuit configuration, large power consumption and high sensitivity to noise.
Also, a circuit using two transistors and two saturable inductors has been used in practice, but its oscillation frequency is too low to provide a frequency of several hundreds kHz.